Mobile networks are currently evolving from pure circuit switched (CS) networks towards packet switched (PS) networks, in particular Internet Protocol (IP) based networks, and by that integrate into IP based infrastructures that are also used for the Internet, the World Wide Web and the datacom industry.
More specifically, technologies allowing voice communication over an IP based network have been introduced. Examples of such technologies are Voice over IP (VoIP) via DSL access or Voice over IP via WLAN access. In mobile networks, technologies allowing voice communication over an IP based network are being introduced as well. Examples of such mobile networks are mobile networks according to the Third Generation Partnership Project (3GPP) technical specifications (TS).
For example, mobile network operators may install IMS (IP Multimedia Subsystem) networks and offer IMS services. It is desirable to make these services available also to subscribers typically using a CS access to the mobile network, e.g. subscribers of 2G and 3G networks such as subscribers of a Global Systems for Mobile Communications (GSM) access or of a Wideband Code Division Multiplex (WCDMA) access. Other examples of 2G/3G access technology are General Packet Radio Service (GPRS), Enhanced Data Rates for GSM Evolution (EDGE) or Enhanced GPRS (EGPRS), Universal Mobile Telecommunications System (UMTS), and High Speed Packet Access (HSPA). Hereto, calls from and to this group of 2G/3G subscribers are routed through the IMS network in order for the IMS service engine to execute the call and call related services. This concept is called IMS Centralized Services (ICS). The IMS centralized Services specifications in 3GPP targets at using the CS access for access to IMS services, see 3GPP TS 23.292, and is complemented by IMS Service Continuity, see 3GPP TS 23.237.
Further, a technology referred to as Evolved Packet Core (EPC) is being introduced as part of the Evolved Packet System (EPS) of 3GPP Long Term Evolution (LTE), supporting a radio access network (RAN) referred to as Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). In 3GPP LTE, voice communication is typically implemented using PS access, e.g. as Voice over IMS or using a technology referred to Voice over LTE via Generic Access (VoLGA), see the VoLGA stage 2 specification available in the internet via “http://www.volga-forum.com”. Another way to implement voice communication in an mobile network supporting with LTE radio access networks is to use Circuit Switched Fallback (CSFB) to a 2G or 3G radio access networks, e.g. to a GSM EDGE Radio Access Network (GERAN) or to a UMTS Terrestrial Radio Access Network (UTRAN). The concept of CSFB is described in 3GPP TS 23.272. A study on the implementation of CS domain services in the EPS can be found in 3GPP Technical Report 23.879. It has also been proposed to implement single radio voice call continuity (SR-VCC), enabling inter domain Handover (HO) of an IMS voice call from PS domain to CS domain and vice versa (see 3GPP TS 23.216). It is desirable that SR-VCC may be combined with ICS for an IMS centralized solution supporting also SR-VCC from PS access via E-UTRAN or UTRAN to CS access via UTRAN or GERAN, and vice versa.
In the following, some aspects of mobility management in 3GPP will be described in more detail. FIGS. 1 and 2 schematically illustrate mobility scenarios in a mobile network with different types of access technology. FIGS. 1 and 2 both relate to a scenario in which the mobile network includes 2G access technology, e.g. GSM access technology, 3G access technology, e.g. WCDMA access technology, and LTE access technology. In FIG. 1, the 2G access technology has a Location Area (LA) and routing area (RA) 200A, see 3GPP TS 23.060, which are separate from the LA and RA 200B of the 3G access technology. The 2G access technology is controlled by a 2G Serving GPRS Support Node (SGSN) 110A, the 3G access technology is controlled by a 3G SGSN 110B, and the LTE access technology is controlled by an LTE Mobility Management Entity (MME) 110C. In FIG. 2, the 2G access technology and the 3G access technology have a combined LA and RA 200A/B and are controlled by a combined 2G/3G SGSN 110A/B. As illustrated by the arrows of FIGS. 1 and 2, a UE may move from 2G/3G access technology to LTE access technology, and vice versa. When moving from 2G/3G access technology to LTE access technology, information concerning an LTE tracking area (TA) 100C, see 3GPP TS 23.401, may be updated by performing a Tracking Area Update (TAU). When moving from LTE access technology to 2G/3G access technology, information concerning the 2G/3G LA/RA may be updated by performing a Location Area Update (LAU) or Routing Area Update (TAU).
To reduce the signaling when moving back and forth between 2G/3G access networks and LTE access networks in idle mode, a mechanism has been implemented which is referred to as Idle Mode Signaling Reduction (ISR), see 3GPP TS 23.401. ISR is mandatory in the terminals and optional for the network. ISR requires an S4-SGSN, i.e. an SGSN with an S4 interface towards the MME. If the underlying 2G/3G network has separate LA/RA for 2G and 3G, ISR is presently only possible between either LTE and 2G or between LTE and 3G, not both at the same time. If the underlying 2G/3G network has combined LA/RA for 2G and 3G then ISR is possible between LTE and 2G/3G. Here, it is to be noted that with combined 2G/3G RAU/LAU it is unknown whether the UE is in GSM or WCDMA/HSPA coverage.
When performing idle mode mobility between LTE and 2G/3G in case ISR is active, a User Equipment (UE) will typically not perform TAU, RAU or LAU, and the UE will not perform IMS (re-)registration. In case ISR is not active, the UE will not perform IMS (re-)registration. When performing idle mode mobility between HSPA and GSM in case of combined RA/LA, the UE might not perform RAU or LAU. It is presently known that the UE performs LAU when the received LAI (Location Area Identifier) is different from the stored LAI, when LAU is needed for a periodic location update, see 3GPP TS 24.008, when VoLGA is used, or when the MSC (Mobile Switching Center) serving the 2G/3G target cell is different from the MSC that served the UE while camping on E-UTRAN, see 3GPP TS 23.272. If neither CS fallback nor VoLGA is being used, the LTE coverage is a “coverage hole” from a CS perspective, i.e. there is generally no location update performed when a UE returns from LTE to CS. If the UE has been CS attached before entering LTE, then the UE will perform LAU when re-entering UTRAN or GERAN coverage only if the LAI has changed or the periodic LAU timer has expired. If the UE has not been CS attached before entering LTE, then the UE will perform LAU when entering UTRAN or GERAN coverage.
The table of FIG. 3 gives an overview on possible terminating call cases for a UE moving between LTE and CS: Here, it is to be noted that corresponding call cases exist if the UE moves between HSPA and CS. In the mobile network, a server termed as Service Centralization and Continuity Application Server (SCC AS), defined in 3GPP TS 23.292 and TS 23.237, decides whether to terminate a call in CS or in PS using LTE or HSPA. In this respect, termination of a call session, or connection refers to the process of supplementing an incoming call, session or connection to the UE by an access leg from the mobile network to the UE, which can be accomplished using PS access or using CS access. In 3GPP TS 23.237 and 23.292, this process is also referred to as Terminating Access Domain Selection (T-ADS). In the table of FIG. 3, “MSC-S registered” means that the MSC-S (MSC Server) is enhanced for ICS and registers the user in IMS.
In the following, termination scenarios in case of idle mode mobility will be further explained. In general, the T-ADS schemes cause the SCC AS to keep track of IMS registrations and the last used domain, which includes the case of an ongoing call. Hence, the following termination scenarios are possible: If the UE is only IMS registered for audio via LTE/HSPA, and UE has used LTE/HSPA for the last call, the SCC AS decides to terminate the call via LTE/HSPA. If the UE is only IMS registered for audio via LTE/HSPA and CS was used for the last call, the SCC AS decides to terminate the call via CS. This may happen after SRVCC when the UE is still IMS registered for audio but the MSC Server has transferred the last call to CS and a terminating call is coming in. If the UE is only IMS registered for audio by MSC Server, the SCC AS decides to terminate the call via MSC Server. If the UE registered for audio both via LTE/HSPA and via MSC Server, the SCC AS decides to terminate the call via the last used domain, the domain last registered in, or according to user or operator preference. If there is no IMS registration of the UE, the SCC AS decides to deliver the terminating call via MSC Server not enhanced for ICS (“break-out to CS domain”). Accordingly, the SCC AS will try first to terminate the call via the last-used domain. In case of termination over PS while the UE is camping on GERAN/UTRAN, either T-ADS by the UE is used or an error case is reported to the SCC AS. If there is no response, a timer expires, or an error case is reported, e.g. as mentioned above, the SCC AS tries the other domain.
Moreover, the idle mode mobility between LTE/HSPA and 2G/3G generally means that, if the UE is attached in CS and has performed IMS registration over LTE/HSPA, it may in addition be registered by the MSC Server. Further, it is to be noted that an IMS capable UE can be registered in the IMS also when using a GERAN or UTRAN access.
However, some situations may arise in which the known T-ADS procedures cause an undesirable delay or result in undesired signalling traffic by the UE. In a first example of such situations, ISR is active and the UE does not perform TAU, RAU/LAU when switching between LTE and 2G/3G. Accordingly, there is a higher likelihood that SCC AS does not know the current access domain of the UE. This may result in an unsuccessful attempt to terminate via PS while the UE is camping on GERAN/UTRAN, and may add delay to the termination process. In a second example of such situations, ISR is not active or not used and the UE performs RAU/LAU, TAU when switching between LTE and 2G/3G. Here, it should be noted that support of MMTel (Multimedia Telephony) on HSPA assumes separate RA/LA for 2G and 3G. In case of LAU, the MSC Server may perform (re-)registration in the IMS as specified in 3GPP TS 23.292, thereby indicating the UE is reachable on CS. If the MSC Server is not enhanced, the SCC AS may rely on CS attach information in the Home Location Register (HLR) to determine whether the subscriber is reachable in CS. The UE could therefore update registration when the access capability for VoIP changes. The UE could also update its access capability for VoIP once it has determined that the used access is not frequently changing. The SCC AS may also use, e.g., Mobile Application Part AnyTime Interrogation (MAP ATI) to check the time stamp of last attach when terminating a call. In the second example, unsuccessful attempts to terminate over PS while the UE is camping on GERAN/UTRAN may be avoided because there is a higher likelihood that the SCC AS knows the current access domain of the UE. However, the signaling load is increased.
So it may happen that the SCC AS has a valid IMS registration for audio via LTE/HS and will therefore try to terminate the call towards the registered contact, but the UE might have been moved into GERAN/UTRAN, and thereby out of the VoIP capable coverage. Then either UE T-ADS has to be used and/or an error case is reported to the SCC AS (see 3GPP TS 23.292). In case of UE T-ADS, the UE may setup a CS bearer or inform the SCC AS about the error condition that the PS domain cannot be used. The SCC AS, if receiving an error response from the UE, will try the CS domain for termination instead.
In either case, the call setup time is increased by the need to establish either a CS originating call leg or a CS terminating call leg if call setup via PS failed.
In case ISR is active the UE will not perform TAU/RAU per TA list/RA. This reduces the signaling with the core network, but at the same time increases the likelihood that the SCC AS does not know the current access domain the UE is camping on. This is especially true in case the UE is in idle mode and not performing IMS registration. When the UE is active in a call, the SCC AS will typically know about the access domain used by the UE.
When now looking closer at the case when ISR is not active, it can be observed that the UE performs RAU/LAU, TAU when switching between LTE and 2G/3G. In case of MMTel support on HSPA, this only applies when there is separate RA/LA for 2G and 3G.
The UE will receive an “IMS voice over PS session supported indication” from MME, SGSN or both per TA list/RA. These indications are valid in the same TA/RA. UE can remember the “IMS voice over PS session supported indication” for each the RA and TA the UE has a valid registration in, independent on whether ISR is used or not.
So if the UE would have the indication that IMS PS Voice is possible over LTE and would move to UTRAN/GERAN and IMS PS Voice is not possible there, then even if the UE would perform RAU then the IMS and the SCC AS in particular have incorrect information about the PS access capability since relying on the IMS registration and hence cannot make a correct terminating domain selection.
As can be seen, the existing solutions are increasing the delay for terminating calls, e.g. as in the first example above, or increase the signaling load, e.g. by requiring the UE to update the IMS registration when access capability for VoIP changes as in the second example above, which may drain the battery of the UE, especially when the UE is in idle mode. In one case, there is a lack of knowledge of the SCC AS with respect to an access capability of the UE, e.g. support of voice communication over a PS access, which may result in unsuccessful termination attempts and increased delay. In the other case, access capability information is made available at the cost of an increased signaling load with respect to the UE, which is undesirable, e.g. in view of battery lifetime.
Accordingly, there is a need for techniques which allow for efficiently handling access capability information of the UE in the presently used access network.